When does prism compound vs cancel?

[ekays1809]

Can someone please help me with this. I've found different answers as to when prism compounds or cancels when it comes to base in and base out prism. According to the "Ultimate ABO Study Guide" by opticianworks, BI/BO - cancels and BI/BI & BO/BO - compound. However when I took the practice test on opticianworks the answer to this question contradicts that.

35. Which of these prism combinations compounds or adds together?
· Base In/Base In
· Base Down/Base Down
· Base Up/Base Up
· Base In/Base Out



And for this question, why are we canceling them when its BD/BU? Wouldn't they compound? I got the right eye as 2.8 BD and the left as 1.5 BU.

38.
· Right eye Rx is -5.75 -1.25 X 180
· Left eye Rx is +5.75 -2.00 X 180
· Customer sat on their glasses and decided to wear them until they could come in.
· Each lens is sitting 4mm high
· How much prism and in what direction did they experience until they saw you?

·1.3 BD R
·4.3 BD R
4.3 BU L
·2.8 BD R
1.5 BU L
·5.1 BU R
5.1 BD L

I'm taking my ABO on November 11th and this is driving me nuts! Thank you guys.

[optical24/7]

Can someone please help me with this. I've found different answers as to when prism compounds or cancels when it comes to base in and base out prism. According to the "Ultimate ABO Study Guide" by opticianworks, BI/BO - cancels and BI/BI & BO/BO - compound. However when I took the practice test on opticianworks the answer to this question contradicts that.

Assuming the prism is the same amount, OU...
35. Which of these prism combinations compounds or adds together?
· Base In/Base In--------------- Compounds
· Base Down/Base Down------- Yoked
· Base Up/Base Up-------------- Yoked
· Base In/Base Out---------- Yoked



And for this question, why are we canceling them when its BD/BU? Wouldn't they compound? I got the right eye as 2.8 BD and the left as 1.5 BU.

38.
· Right eye Rx is -5.75 -1.25 X 180
· Left eye Rx is +5.75 -2.00 X 180
· Customer sat on their glasses and decided to wear them until they could come in.
· Each lens is sitting 4mm high
· How much prism and in what direction did they experience until they saw you?

·1.3 BD R ----incorrect
·4.3 BD R
4.3 BU L
·2.8 BD R -------correct
1.5 BU L --------correct
·5.1 BU R
5.1 BD L

I'm taking my ABO on November 11th and this is driving me nuts! Thank you guys.

You are correct. If not, I'd fail too!

[Robert Martellaro]

The primary effect on the eyes is best described as a vertical imbalance of 4.3∆ base down Right or 4.3 base up Left.

[optical24/7]

Robert is correct in how to term the effect as total imbalance ( I had the 4.3's highlighted as correct ) then edited it to say what each eye was experiencing. There are several different correct answers there, just the wrong one marked correct on the test.

[obxeyeguy]

...

[John@OWDC]

ekays1809:

Why did you post this here instead of asking us at Laramy-K?



Anyway, yep, mistake on my part, my bad, me wrong, I'll fix it in a few minutes.

Thanks for catching that and I will fix it!

Sorry! [red face emoticon there]

Anything else like that just let us know directly and I'll fix it ASAP.

John

[Diane]

Rule is vertical in same base direction cancels, horizontal in same base direction compounds.

[ekays1809]

Thank you everyone!!! One more question! Does anyone know if the practice test on the ABO website is a good idea to take or has anyone ever taken it? Or am I just as well off taking the practice tests on laramy-k?

[John@OWDC]

I would do both.

[Uncle Fester]

And for this question, why are we canceling them when its BD/BU? Wouldn't they compound? I got the right eye as 2.8 BD and the left as 1.5 BU.

38.
· Right eye Rx is -5.75 -1.25 X 180
· Left eye Rx is +5.75 -2.00 X 180
· Customer sat on their glasses and decided to wear them until they could come in.
· Each lens is sitting 4mm high
· How much prism and in what direction did they experience until they saw you?

·1.3 BD R
·4.3 BD R
4.3 BU L
·2.8 BD R
1.5 BU L
·5.1 BU R
5.1 BD L

I'm taking my ABO on November 11th and this is driving me nuts! Thank you guys.

May I nit pick?

What is meant by "each lens is sitting 4mm high"?

How does one sit on their glasses and make the lenses sit 4mm high?

What's the assumption my feeble mind should be grasping???

Did the nose pads move closer together causing the OC's to rise up 4mm?

Or is the exact wording different?

[optical24/7]

I think it's just a story line to get to the question. It could have read.....

" Patient with the above Rx is sitting under a tree. A squirrel pees on him. He looks up to see what peed on him and was looking 4mm above his OC's. How much prism and what direction was he experiencing?"

[John@OWDC]

Yes, you may nit-pick and yes you are to assume that they have a very, very uniquely shaped bottom, they were about to sit on an oddly shaped chair and their sitting on their glasses resulted in the nose pads being squeezed together, thus raising the lenses up as they sit in front of their eyes.

Good point, I'll reword that one.

Thanks

John

[optical24/7]

Yes, you may nit-pick and yes you are to assume that they have a very, very uniquely shaped bottom, they were about to sit on an oddly shaped chair and their sitting on their glasses resulted in the nose pads being squeezed together, thus raising the lenses up as they sit in front of their eyes.

Good point, I'll reword that one.

Thanks

John

I like my example better. It was on my AC test. I got it wrong...................I answered, " He saw two squirrels peeing on him"...............